1. Technical Field
The present disclosure relates to an integrated device of a capacitive type for detecting humidity, in particular manufactured using a CMOS technology
2. Description of the Related Art
Presently, devices for detecting humidity or hygrometers are widely used for a variety of applications, including controlling humidity of industrial, agricultural, and living environments. In particular, in view of the current trend to miniaturization and the preference for arranging small dimension hygrometers in portable apparatuses, a desire is increasingly felt of having integrated hygrometers of very small dimensions.
To this end, integrated hygrometers manufactured using a semiconductor technology have already been proposed, since they have good accuracy, based on exploiting the ability of a hygroscopic material to change an electric property of the device as a function of absorbed water particles. In particular, polyimide layers have been proposed, since they undergo a linear variation of their dielectric constant ∈ps as a function of relative humidity, according to the behavior shown in FIG. 1.
Electronic chips forming capacitive-type humidity sensors have been already disclosed (see, e.g., U.S. 2005/0218465 and H. Shimizu, H. Matsumotu et al: “A Digital Hygrometer” IEEE Transactions on Instrumentation and Measurement, Vol. 37, No. 2, June 1988). The known devices include a sensing capacitor formed in a sensing layer on top of a silicon substrate and having metal fingered electrodes. A hygroscopic layer of, e.g., polyimide, covers or overlies the metal fingered electrodes of the sensing capacitor. The top hygroscopic layer is thus able to capture water particles of the external environment and to change its dielectric constant as a function of environmental humidity. Thus, also the capacity of the sensing capacitor varies, and the variation can be read by a suitable circuit, including, e.g., an input capacitive bridge.
Humidity sensors may be formed of the single and differential type.
Both types are however susceptible to improvements. In fact, the single-type humidity sensors typically utilize a small area, but are affected by matching inaccuracies and ageing. On the other hand, the differential-type humidity sensors are less affected by matching and ageing but utilize a much bigger area and are subject to leakage.
In particular, the differential solution may be based on forming a reference capacitor near the sensing capacitor and having the same structure as the sensing capacitor, but for the hygroscopic layer. In one solution, the reference capacitor may have no hygroscopic layer at all; in another solution, the reference capacitor may be shielded from the external environment.
In both cases, the reference capacitor is intended to be insensitive to humidity changes but follow the behavior of the sensing capacitor in all other aspects, so that the variations in the electric characteristics due to ageing, temperature, manufacturing spread and so on are the same, so that a reading circuit is able to detect property changes in the sensing capacitor caused by changes in humidity of the external environment and to distinguish them from other effects.
However, with the increasing miniaturization of the integrated devices, both solutions are not sufficient to ensure the desired insensitivity to humidity.
In fact, in the first case (where the reference capacitor has no hygroscopic layer), the latter has to be removed from the reference capacitor after being deposited on the entire surface of the wafer. However, the absence of the hygroscopic layer on the reference capacitor weakens the structure, because it involves forming an aperture in the layer, thus impairing its function as a mechanical protection and as humidity barrier. In addition, removal is costly and critical, since removal of the hygroscopic material from adjacent, sensing areas is to be avoided and limits the desired miniaturization.
On the other hand, the shielding solution has proven insufficient. Shielding may be obtained by depositing a humidity blocking layer of a different non-hygroscopic dielectric material over or under the hygroscopic layer. However, with this solution, the reference capacitor cannot be made insensitive to external humidity, as also recognized in U.S. 2005/0218465. Similar problem are encountered if the shielding layer is arranged between the electrodes and the hygroscopic material. In fact, with horizontal miniaturization of sensors, electric field lines exiting from an electrode finger and closing in an adjacent, oppositely biased electrode finger have a considerably vertical extension and may reach the hygroscopic layer, so that the reference capacitor is not insensitive to humidity. To avoid this, a very thick shielding layer may be used, with thicknesses of more than ten microns. Such thicknesses cannot be reached with standard machines and deposition techniques, which currently allow deposition of dielectric layers having standard maximum thicknesses of about one micron. Thus, no efficient shielding can be reached with either solution.